In-Loop Filter - Advanced Video Coding Systems - page 128

Fig. 7.11 The region division of WVGA sequence
picture_alf_enble[ i ], control whether ALF is applied for the corresponding i th image
component. Only if the picture_alf_enble[ i ] is enabled, the corresponding LCU-level
flags and filter coefficients for that color component will be transmitted. The LCU-
level flags, lcu_alf_enable[ k ], control whether ALF is enabled for the corresponding
k th LCU, and are interleaved into the slice data. The decision of different level
regulated flags is all based on the rate-distortion cost. The high flexibility further
makes the ALF improve the coding efficiency much more significantly.
7.4.3 Virtual Boundary Processing
The filter unit of ALF is LCU and normally the filter process starts from the top-left
sample of the LCU and ends at the bottom-right sample. However, the filter process
cannot be done until the LCU below has been reconstructed. Hence extra lines of
buffer will be needed for ALF, which requires large chip areas. In order to solve
the problem, virtual boundary (VB) process is proposed to ALF (Tsai et al. 2013 ;
Zhang et al. 2014b ), which divides one LCU into two parts, i.e., parts below virtual
boundary and above virtual boundary. For the part below VB, four lines of buffer
has to be spared, where there are three lines for deblocking process and one line
for SAO process in Fig. 7.12 . The samples in part below VB cannot be used in the
current LCU filtering process, because deblock filter and SAO cannot be applied
for them. Thus, they are left without filtering in current LCU process. For the part
above VB of current LCU, there are already four lines samples buffered for SAO
and deblocking filter and those samples can be used in the ALF filtering process.
Hence, the present ALF processing start boundary is no longer the above boundary
of current LCU. The ALF processing boundary of current LCU should be shifted to
the VB of the above LCU, which is not included in the filtering process of the above
LCU, illustrated in Fig. 7.13 . There are still some samples, between VBs in current
LCU and above LCU, which are filtered with cross-virtual boundary samples. For
these samples, AVS2 uses takes the nearest available samples to replace the samples